Tempered glass sheet provided with a transparent electrically conducting film



Dec. 28, 1954 G S R 2,698,261

TEMPERED GLASS SHEET PROVIDED WITH A TRANSPARENT ELECTRICALLY CONDUCTINGFI LM Original Filed Nov. 10, 1949 3 Sheets-Sheet 1 Junentor//////;SZ//////////-I H Dec. 28, 1954 R. A. GAISER 2,698,261

TEMPERED GLASS SHEET PROVIDED WITH A TRANSPARENT ELECTRICALLY counucwmcFILM Original Filed Nov. 10, 1949 3 Sheets-Sheet 3 fig. if

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dtfomcga United States Patent giRLAJFSPARENT ELECTRICALLY CONDUCTINGRomey A. Gaiser, Toledo, Ohio, Owens-Ford Glass Company, ration of OhioOriginal application November 10, 1949, Serial No. 126,521. Divided andthis application November 13, 1951, Serial No. 256,101

1 Claim. (Cl. 117-124) assignor to Libbey- The present invention relatesto vitreous bodies provided with electrically conducting surface films.More particularly, it has to do with a specially heat strengthened glassplate having a transparent, electrically conducting film on one surfacethereof.

This application is a division of my copending application Serial No.126,521, filed November 10, 1949.

It is already known to provide a transparent, electrically conductingfilm (such as shown in Patent 2,429,420 to H. A. McMaster, dated October21, 1947) on glass sheets or plates, by first heating the sheets tosubstantially their point of softening and then spraying one surface ofthe hot glass with a tin halide in fluid form; after which the filmedsheet is cooled in the atmosphere.

However, the strength and breaking characteristics of filmed glasssheets produced in this manner are little, if any, better than those ofordinary annealed glass. In fact, it is said that such filmed sheetsbreak like poorly annealed glass.

Now, due to the expense of producing electrically conducting glasssheets of this character, it is important that they stand up under morethan average rough usage, and this is particularly true because atpresent the widest'use of these conducting sheets'is in de-icing windowsor windshields of airplanes, where they are subjected to veryconsiderable abnormal strains and stresses resulting from their use athigh altitudes and in pressurized cabins.

It is therefore an importantaim of the present invention to produce asheet or plate of glass that is filmed with a transparent'electricallyconducting film and which, at the same time, will exhibit mechanicalstrength and breaking characteristics under predetermined pressureToledo, Ohio, a corpoloads that are as good as, if not better than,those of fully heat treated, or full tempered glass.

Briefly stated, this is accomplished, according to the invention, byfirst heating'the glass sheet to substantially its point of softening,removing it from the furnace and,

preferably after a few seconds delay, spraying one side of the hot sheetwith a filming fluid while at the same time quickly cooling the oppositesurface. a

Another objectof the invention is to produce afilmed and speciallystrengthened glass sheet by spraying one surface of a heated glass sheetwith a solution containing a tin halide, or other filming material,while simultaneously blasting the opposite surface with moving streamsor jets of air. v Another object is to controlthe thickness of thecompression layers set up at opposite sides of the glass sheet bycontrolling the degreeof cooling exerted by the filming spray on oneside, and the moving air on the other,

during the filming and tempering treatment.

Another object is to prevent the filming material from reacting with thesurface of the hot glass sheet opposite to the surface being filmedduring the filming procedure.

Other objects and advantages of the invention will become moreapparentduring the course of the following description, when taken inconnection with the accom panying drawings.

In the drawings, Whereinlike numerals are employed to designate likeparts throughout the same:

Fig. 1- is a side elevation of the exit end of a heating furnace, andtheassociated apparatus for filming and strengthening the glass afteritleaves the furnace," as constructed in accordance with thepresentinv'ention;

Fig.2 is a section taken substantially along the line f: I l I v Fig. 3is a vertical sectional view along the line 3-3 in Fig. 1

Fig. 4 is a horizontal sectional along the line 44 in Fig. 3;

Fig. 5 is an enlarged fragmentary, vertical sectional view showing thedetail of the air blast pipes and the air exhaust means;

Fig. 6 is a fragmentary plan view of the glass supporting mechanism;

Fig. 7 is a fragmentary section through a glass sheet that has beenspecially strengthened and filmed in accordance with the invention;

Fig. 8 is a similar View of the glass sheet of Fig. 7, showing the usualmanner of combining a glass sheet provided with a transparent,electrically conducting coating with a second sheet of glass and aplastic interlayer to form laminated safety glass;

Fig. 9 is a fragmentary edge view of a glass sheet which has beensubjected to substantially equal cooling on both surfaces during thefilming treatment, showing diagrammatically how the strain pattern insuch a sheet appears when viewed with a Babinet compensator (this strainpattern is substantially identical with the pattern seen when a fullytempered glass plate is viewed in the sam taken substantially view takensubstantially y manner);

Fig. 10 is a similar view of a glass sheet which has been filmed withoutthe special tempering treatment of the invention;

Fig. 11 is a similar view of a glass sheet which has been subjected togreater cooling on the unfilmed surface and lesser cooling on the filmedsurface during the filming treatment; and

Fig. 12 is a similar view of the glass sheet of Fig. 11 when bent underthe pressure load to which such sheets are subjected in aircraft use.

Referring now more particularly to the drawings, there is illustrated inFigs. 1 to 6 a preferred form of filming and strengthening apparatusincluding a furnace A and a v filming and strengthening mechanism B. Asbest shown in Figs. 1 and 2, the furnace A is of the rectangular tunneltype comprising side walls 10, bottom wall 11, and top wall 12 providedwith a continuous middle slot 13. The furnace may be heated in anydesired manner such as by means of electrical resistance heaters 14along the side walls, and is preferably provided with baffle plates 15in front of the heaters 14 to provide a more uniform heat in the workingarea of the furnace.

In order to move work through the furnace A and to transfer it from thefurnace to the filming and strengthening apparatus B,- there is provideda monorail 16, mounted above the slot 13 in the furnace A by brackets 17and extending outwardly from the furnace to and beyond the I filming andstrengthening apparatus B.

Movably supported from the monorail 16 by wheels 13 is a carriage 19provided with link members 20 extending downwardly through the slot 13in the furnace A and carrying at their lower end tongs 21 from which aglass sheet 22 can be hung.

A glass sheet 22 to be treated, after being hung from the tongs 21 atthe entrance end of the furnace (not shown), is introduced into, andpassed through, the furnace A- by movement of the carriage 19 along themonorail 16. The speed of travel of the carriage and the tem-,

peratures within the furnace are so controlled that by the time thesheet 22 reaches the exit end 23 of the furnace A. V it has been heatedto substantially the softening point ofthe glass. The furnace door 24 isthen opened and the,

glass sheet ismoved therethrough and into position to be acted upon bythe filming and tempering apparatus B.

The apparatus B includes a filming device C, positioned to one side ofthe path of travel of a glass sheet from the furnace, and a coolingdevice D, located at the opposite side of the path of travel of theglass, between which the heated glass sheet is positioned forsimulshafts 29 which are; carried by bearings 30 hung from H supportedfrom the shafts 33 by bearings 36 within-which thesnafts arejournaled'and'to'the bases of which thc platform is bolted. 7

Depending from the platform is a vertical -rod 37 upon which theindividual spray guns 88 of the bank 25 are supported. Tothis end, thebody portion'of each of the guns '38 is provided with a conventionalclamping collar 39 by means of which the guns can be adjustablyclamped-to the rod 37 in desired vertical relation to one another and'atoneside 'or the other of the rod. i

In order to cause the spray guns to reciprocate or sweep back and forthacrossand in a plane'parallel with the glass sheet during the filmingprocedure; there is provided a drive meansfor the frame 26 whichincludes a motor 40 mounted on aplatform '41 that is hungifrom'the beams32 by a framework 42. The motor 40drives a gear reducer 43, also mountedon the platform41, 'andwhich' has keyed-to its driven shaft 44 aneccentric member 45. Positioned directly abovethe eccentric member '45isa vertically disposed, circular crank plate "461 rotatably mounted on'an axle 47 which is'carried in a bracket 48 secured to the framework 42.

The crank'plate 46 has driven connection with the eccentric member bymeans of a rod 49, secured at one end to the lower periphery of theplate 46 by a pin 50 and at its opposite end to the face of theeccentricmember 45 by a pin 51; and driving connection with a crosslink'52, running between and secured to the pair of bars 27 with studand bracket connections53,by'meansof a rod'54 secured at one end to theupper perimeter of the plate 46' by a pin 55 and at its opposite end byayoke and pin connection 56 to the end of the cross link 52.

Thus, upon operation of the motor'40,'theeccentric member 45 will be.rotated through, the speedreducer 43 to impart a crank motion to'theplate 46 which in turn will'cau'sethe rodS land cross link 52 to bereciprocated andthus impart 'a'swinging' pendulum-like motion to theframe 26carrying thes'prayguns '38. The length of the penduluin=likeswing of theframe26 can ofcourse be made shorter or longer by moving thepin SSWhiCh secures the end of the rod to the crank plate 46 to one ofthe other holes 57 in'the'plate.

The cooling device D,which is positioned in substantial alignment withthe spraying device C on the opposite side of the path of travel of theglass sheet 22-from the furnace A, comprises a vertically arranged,substantially'rectangu-. lar exhaust chamber "58 eonstructed of sheetmetal and provided with a'stack' 59 leading to'suitable' air exhaustingmeans (not shown). ArrangedWitbin openings 60' and 61; in the front andback Walls respectively of the'chamber 58 is a horizontally arranged opeended, sheetm'etal casing 62extending entirely through the chamber58andprojecting outwardly beyond the front and rear thereof. A plate 63,which .is welded othe ca ing 62" diacent' its rearenclandlprojectslaterally'from all four sides thereof,

is bolted to'the back w ll of'the 'chamber 58 to mount the casing in thechamber and to close :theportions of the ope 1ng61- that surroundstbec'asing. The passage way 64'throu'g'h the casing 62'is then directlyonnosite to, and in direct alignment with,-the spray'guns 38.

Formed integr llvavithg orwelded' tofthe'outside of thefour walls of'thecasing 62,atitsfforward' end. is a lateral y ex endin surrounding wall65 provided with a relativelyfshort straight portion 66 running parallelwith a d outside of theside wa ls of the chamberr58 and a relativelylonger portion 67 angled r earwardly toward and extending into thechamber 58 through the opening 60 in the 'frontwall thereof; A'correspondingly shaped, laterally extending wall-681s positioned inspaced parallel relation to-the wall 65 by spacer blocks-69 to providea. passageway 70leading from'an area 71, "outlined by'the walls 65 and68; directly in front'of the casing 62, "to the" interior of the exhaustchamber 58. A cover'plate 72 which-'is similar to the eoverplate' 63 isweldedto the wall 68- 'to' support'the sameand to close the portion "ofthe opening al-surrounding" the casing-62' This eonstruc-'tiorr'perrnits the wall 68-to be readilyremoved from' association withthe casing 62, for reasons tobe more clearly hereinafter-set forth;while; at* the same time,

4. assisting in supporting the casing 62 within the chamber 58L Themeans for cooling the'glass sheet 22, when in filming and strengtheningposition,.is carried by a base 73 located on the bottom wallof thecasing 62, and may comprise a plurality of vertically arranged, spaced,cooling ends of theistrap 76' The rods 77 are mounted; adjacenttheirforward'ends, on oppositeends of a shaft78 slid'able withinbrackets 79 secured to the base73; and the.rear-' ward ends of the rodsare fixed in blocks 80, also secured to the base 73.

in order to impart the desired reciprocatory movement to the pipes74t'here is-provided a motor-'Slr'nounted on the base 73 andhaving-drivingconnection'with a gear reducer82 through the motordrive-shaft 83 to drive an eccentric'84Lcarried'at the'end-of thedriven-shaftSSI The eccentric 84 is operatively connected, by means of arod 86, with one end of' a horizontally arranged bell crank 87-whichis'mounted'on a vertical shaft 88 extending upwardly. from a bracket 89fixed to the base 73. The opposite end of-the bell' crank-87 has aforked portion 90 which surrounds a pin91 extending upwardly from theshaft 78 midway its'ends.

Upon operation of the motor 81, theeccentric 84 will be rotated, and themovement'of the eccentric will be translated through the 'rod'86'andbell crank 87 to'the shaft 78 causing it to slide back andforthwithin the brackets 79 and thus rapidly reciprocate the pipes 74withinthe casing 62'against the flexibility of the rods 77 which act as springlevers'to accelerate the reversal of movement of the shaft 78.

With the particular "construction 'of cooling means D as shown in thedrawings, and'which in'cludes the laterally extending walls 65 and 68,"it is'necessary' that the glass sheet be moved laterally to bring itinto proper position for filming and'cooling'after it-has beenmovedalong the m n rail 16 fr m the furnace A and into position b tweenthedevices C'and D. To this end, the carriage 19 includes a horizontalsupporting plate 92 provided'with transverse slots 93; The'lin'k'members 20 are supported from the supporting plate by pins 94passing through,

theslots 93 and which are provided withheads 95 that engage theupper'surface of the'plate 92.

During'movement'of the carriage "19 to "carry' the' sheet throughthefurnace and to 'bring it intoposition between the spraying device Cand the coolingdevice'Djthepins 94'are preferably midway'of the'ends ofthe slots 93' and directly below the'monorail 16 in'order tobalancetheglass 'sheet suspended therefrom 'on' the carriage." Then, as thecarriage arrives i'nfront of'the'cooling'device D,"

the pins 94, carrying the link members 20, are .moved in glass is readyfor final treatment.

I prefer :to allow 'a' timezlapse of :severaltseconds be tween thetimethe glass sheet=22 .is removedzfrdmrthe: furnace and the 'itime'lwhendtis subjectedi to 'the'action of the filming 'spray*andthecooling air;because I have found that such a time interval 'hasanirripo'rtantbearing on the'finished filmed and tempered sheet.

With the glass sheet in position for filming and cooling,

the motor 81 is started to reciprocate'thepipes '74 back and forthacross and in close proximity to 'therear surface of'the'glass'sheet 26.Simultaneously;compressed Y air is fed to the header 75'through aflexible-conduit. 9.8 connectedthereto andleadin g to La. source .ofs'upply L (not shown), which air-is distributed-tothe-several.pipes .74and forced out againstthe surface of. the' glass .through vertically'spaced'openings:=99 therein.- This ..results ins theirear'surface. ofthe: glass being. quickly chilled: -by moving blasts-or 'jets .ofrairrimpinging'ionirand vsweepingfir;

over its surface.

At the same time, the cooling is begun, or zpreferablyi immediatelyafterward, the motor is started to swing the spray guns 38 back andforth across the front surface of the sheet 22. Simultaneorsly, the guns38 are turned on by admitting air through pipes 100 into an air cylinder101 in the body of each gun to retract the piston rod 102, connected tothe conventional needle valve (not shown) of the gun, and which valve isnormally held closed by spring pressure. Opening of the needle valvepermits air under pressure which is supplied to the guns through pipes103 to atomize the spraying liquid, fed to the guns through pipes 104from containers 105 on the stationary latform 41, and to blow itoutivardly against the sheet 22.

It will be noted that the opening 97 in the wall 68 is of the same sizeand shape but slightly larger than the area of the sheet 22. This isimportant in connection with the spray filming of the sheet and in theremoval of the surplus spray with the special exhausing apparatus shown.For this reason, in treating sheets of different sizes and shapes, it isdesirable that a correspondingly sized and shaped opening be provided inthe wall 68. This can readily be done by removing the cover plate 72,which carries the wall 68, from the chamber 58 and replacing it with adifferent cover plate carrving a wall which has the desired size andshape of opening.

Heretofore, considerable difficulty has been experienced, in sprayingglass sheets to produce electrically conducting films, because of thefact that some of the surplus spraying liquid that is being applied toone surface of the sheet is inadvertently deposited on the oppositesurface which it is not desired to film.

According to the present invention, there is provided a special type ofsuction apparatus which immediately removes sur lus spray from the sceneand effectively prevents filming of the rear or unsprayed side of theglass under all circumstances.

Specifically, this suction means comprises the spaced walls and 68 whichform the passageway 70 leading from the area 71 in which the sheet to betreated, and the exhaust chamber 58 into which the passageway 70discharges.

Since air is being continuously. exhausted from the top of the chamber58, through the stack 59, a draft will be set up through the passageway70 to continuously pull a stream of air from all four sides of the area71 around the glass sheet 22 as best shown'by the arrows in Fig. 5.Consequently, anv surplus of the spray 106 to the sheet 22 will beimmediately entrained in the flow of air moving into the passageway 70and exhausted through the stack 59. At the same time. because the sheet22 substantially closes the opening 97 in the wall 68, a stream of airwill be pulled in through the opening in the casing 62 against the rearsurface of the glass sheet.

This continuous wash of air over the sheet surface and toward the edgesthereof further assists in preventing any film formation on this rearsurface and, even when the blower pipes 74 are not used, will still actto chill the heated glass by the impact of moving air in contacttherewith and thus impart some tempering action to this surface of thesheet. However, in order to obtain full strength and best breakingcharacteristics, I prefer to chill the rear surface of the sheet by airblasts from the reciprocating pi es 74. Moreover, the washing action ofthe air from these pipes is so effective in preventing film formation onthe rear surface of the glass that the special exhausting means can beeliminated, if desired, when the air pipes 74 are used After the glasssheet 22 has been positioned within the area 71 and sprayed with filmingsolution on one side and blasted with air on the other for a sufiicientlength of time to produce the required film on one surface of the sheetand the desired degree of temper in the glass. the link members 20 areagain moved back to the middle of the carriage 19 and the carriage movedto the right along the monorail 16 to unloading position (not shown),where the sheet 22 which is now provided with a film 107 (Fig. 7) can beremoved from the tongs 21 and is ready for use.

It will be understood that many, if not all, of the features of thepresent invention will be valuable for use in connection with theproduction of any number of different kinds of sprayed films. However,to date, the invention has found its greatest commercial value in theproduction of glass sheets or plates filmed with a transparent,electrically conducting coating of the general type dis- J to theopposite side or surface chanical strength in use than fully closed inthe patent to Harold granted October 21, 1947.

To illustrate the technique, in a preferred manner of:

apparatus. The time interval between removal of the glass from thefurnace, and the application of filming solution to one side of theheated plate and air to the other, is scheduled to give a definite timelag between these two operations. In the present commercial productionof this particular light, the time lapse between the heating and thespraying and tempering operations has been set at 5 seconds, plus orminus one second.

A spray solution of 10% stannic chloride in isopropyl alcohol by volumeis employed, and one side of the sheet is sprayed with this solution atthe rate of 600 to 800 cc.- per minute while at the same of the glassfrom the reciprocating air pipe mechanism. The air pressures used areapproximately 50 pounds per square inch for the spray solution andapproximately pounds per square inch for the air blast. With glass ofthis size I locate the reciprocating pipe mechanism approximately 6inches from the glass surface and the spray gun heads approximately 8 to10 inches from the glass surface.

Obviously different temperatures and healing times, in

the furnace, as well as different pressures of air and;

spray, and different spacings of air and spray mechanisms, may, andoften are, used in treating lights of other size,- shape and/orthickness in order to produce the desired end result. Moreover, all ofthese factors can be varied relative to one another in a manner to stillproduce the same result. 1

Different filming fluids can also be employed, such as solutions ofvarious tin halides, and even vapors of filming materials such asstannic tetrachloride, stannictetraiodide, stannic tetrabromide, etc.,all of which can i be sprayed on hot glass to trically conducting films.Furthermore, non-filming fluids in liquid, semi-liquid, or gaseous formcan'be used instead of air to chill the unfilmed surface of the glass.Examples of these are steam, oil and mists of water-oil emulsions.

The important points are, when transparent, electrically conductingfilms are to be produced, that the glass be heated to substantially itspoint of softening, that the two surfaces of the heated sheet be treatedsubstantially simultaneously, one by a moving, non-filming fluid, andthe other by moving sprays of filming fluid for a sufiicient length oftime to produce the desired thickness of film on one surface and thedesired degree of temper at both surfaces, and, preferably, that therebe a definite time produce transparent, eleclapse between the heating,and the filming and final strengthening treatments.

As explained above, by the technique of this invention, it is possibleto produce a filmed sheet or plate of glass having as great and, in somecases greater, me-

tempered plate glass. Fully tempered plate glass has been generallyconsidered the strongest form of tempered glass, and is produced byfirst heating the glass to substantially its point of softening and thenrapidly and uniformly chilling the opposite surfaces thereof to placethe exterior of the plate under compression and the interior undertension. Glass plates treated in this manner have their mechanicalstrength greatly increased, and are also given a predetermined breakingpattern so that, if and when broken, they will not splinter likeordinary glass plates but will disintegrate uniformly into innumerabletiny fragments which resemble the crystals of rock candy.

In other words, the added strength and desirable breakingcharacteristics of this glass are the result of intentional andcarefully controlled strains that are set up in the glass by thetempering procedure and, when the edge of such tempered glass sheets isviewed with a Babinet compensator the strain pattern will appear asshown at 108 in Fig. 9, and the exposed surfaces of the glass will be incompression, balanced by the tension inside.

As explained above, when electrically conducting films are applied bythe procedure heretofore generally employed, and in which the heatedglass is sprayed with A. 'McMaster, 2,429,420,

F. furnace for 3.25 minutes and is then positioned between the filmingand chilling time an air blast is applied filtnin'g h'quid v onronecl.side only," iwithlzthemther :side.--,exposed to substantiallyquiescentair atroomitemperature, its. strength and; breaking.characteristics were. :similar. to thoseof poorly.:tempered glass.lrhaveldiscoveredz that this 1511116 tOsth ....fact thatstrains-are.introduced-intothe' glass bythe spraying procedureand.-;that.such strains are:11on-uniorm-and result in axmuch thickercompression layer-on::the..filmed' side of..the glass than onntheunfilmed side, as shown inthestressdiagram fora sheet filmed 11111111115manner in: Fig, 10.

.:This is,1of course,'undesirable; butit is especially so in thencase;of glass-asheets:filmed with an electrically conducting; coating ;which.have found their greatest commercialuse in,the glazing ofaircraft.Thus, for". this purpose it is desirable tQ-cQm-biner thencoated:.sheettogether. with at .least. one .l other sheet .of 1 glass itlfi andanuinterposedrjlayer of. plastic materialzllO. into .ahlami'nated-.:str.ucture .(Fig.-z8). :When;this istdone, it is .considered:necessary to ;place t'rthe coatedv sheet with: theelectrically:,conducting:.film. 1071 to the inside, and. incontact.:with...the.plastic-.interlayen;as. shown in,Fig. 8, in:.orderto avoid. the: :possibility: of shocks: tfromt'and short circuitsin.:th'e: fi1m, and to locate the filmed. sheet 22' asthe'. outboard"light toiobtainithe moshettective deicing'action. .Consequently,:theside ofrthe glass. having the rthinnest;compression. layeris;placedinzitension during pressurization-and fails atbelowthearequired levels.

According tothe'present invention, however, the heated glass'ShCt",WhiCh has one surface chilled by thefilming spray may at. thesamertime have its opposite surface chilled inthe same degree by blastsof cooling air from the reciprocating 'air pipes 74. This results in afilmed and: strengthened sheet having a stress pattern as shown in'vFig.9.: Or, in-other words, ,one'that is substantially the same as fullytempered glass, and in which the compression layers. on the oppositesides are of equal thickness. 'Whentreated inthis manner,the'resultan't'filmed sheetwill have a mechanical. strength, andpredetermined breakingizcharacteristics,that are substantially identicalwith that of fully tempered plate glass.

As a modification; ofthe invention,.the filmed sheet maybe given amechanical strength-under certain :conditions, that is actually greaterthan that of fully tempered glassqunder the same conditions.

To illustrate, treatment,,the;rearor; unfilmed .surfacemay' be chilledto a greater degree than. the surfacebeing filmed. This canbe'accomplished'by starting the air blast against the rear:surfacebefore starting the spraying of the front during the filming..and strengthening 4 8 surface,-- byrincreasing, thervolume of.cooling. airblasted against .theerearsurface, bytheuse of lowerair.temperatnres.,-.etc.

. However. accomplished, .the resultantfilmed .sheet .will

I have a stress pattern as shown in Fig. 11, in which the compressionlayeron. thennfilmedside is thicker than the compression/laymen thefilmed side. Now when such .a filmed sheet iswmounted in an airplane inthe conventional manner, that is, with the filmed side ofthe sheettoward the: inside of .the plane, and, in the. case of laminated safetyglass, towards-theinside of the lamination, the glass will, underpressurization, bend in :the direction shown in Fig. 12.

-When sobent, the applied.stress.:combined:with theinternalstresswillgive the distribution shown by-the strain linesv 111in Fig-.12; where the compression of the. opposite surfacesis...equal:and is. balanced by the tension inside...the.shcet;LConsequently,. the stress diagram of this particular. .filr'nedzvsheet,under'load; will be .thesame as thelstres s pattern of arfully temperedglass plate when not.under load (Fig. 9), :with. the. resuit that .a,light so. treated exhibits maximum mechanical strength .nnderpressurization and so is stronger than a fullytempered plate .under suchconditions.

It is to be understoodthat the form of the invention herewith shownanddescribed is to be-talren as an illustrativeembodiment only of..thesame,. and that .various changes in .the shape, size. and arrangement ofparts, as well as. variousproceduralchanges mayv be resorted to withoutdeparting from .the .spirit of the invention or the scope of thesubjoined claim.

I claim:

.As a new article ofmanufacture, a sheet, of glass having-compressionlayers on oppositesides thereof and its interiorunder tension, and atransparentelectrically conducting film on one surface of said. sheet,said .sheet being. .fur.ther characterized by. the vfact thatv thecompression layercnthe unfilmed'side thereof isthicker thanthecompression layeron thefilmed side.

' References Cited in the file of this patent UNITED STATES PATENTSNumber Name .Date 2,194,611 Paddock et a1. ,Mar. 26, 1940 2,236,911 LongApr. 1, 1941 2,311,846 .Littleton et al. Feb. .23, 1943 2,564,707.Mochel. Aug. 21, 1951 2,584,859 Gaiser Feb. 5, 1952

